Fort Dodge Messenger, IA
04-11-07
Extreme freeze hurts alfalfa plants
By Kristin Danley-Greiner, Farm News staff
Alfalfa producers should soon know if the recent rash of cold weather damaged
their new crop.
Steve Barnhart, forage grass and legume management expert with Iowa
State University Extension, said the range of damage could go from newly
emerged regrowth shoots frozen back to the ground and plant development up to
the first cutting being delayed a week to 10 days, due to the plants needing to
mobilize another set of crown buds for a second regrowth,î to extreme freeze
damage and a kill of most of the alfalfa plants in large areas of the Upper
Midwest states. Essentially, it could cause “a very significant hit to the national
hay supply,î” he said.
“An assessment of the fields in a week to 10 days will be telling,”î Barnhart said
Monday.
Barnhart said well established, regrowing forage plants have lost most of their
winter cold hardiness and that the exposed tissue is susceptible to cold
temperature injury.
“Several hours of 24 to 25 degrees Fahrenheit temperatures will damage leaf
tissue and may seriously damage buds and growing points. If recovered plants
are several inches tall, low 20-degree air temperatures will likely damage one to
several sets of trifoliate leaves that were exposed at the top of the canopy,î” he
said. “The buds and growing stem tips are somewhat more protected and often
continue to grow normally following moderate frost events. One of the most
difficult decisions in alfalfa scouting is whether these temperature ranges have
damaged the crown and taproot tissue—a more serious physiological plant
concern.î”
Auto-temperature recording weather stations across Iowa recorded the low
temperature at the four-inch soil depth in the mid to high 40s a week ago
Monday, with low 30s in southern and northeastern Iowa. It even reached 27
degrees in northwestern Iowa. But Barnhart said the remaining unknownî is the
soil and crown tissue temperatures at 1-inch and 2-inch depths where the active
crown tissue is located.
At emergence, alfalfa and most winter-hardy forage grass and legume seedlings
are extremely cold tolerant,î Barnhart said. But, spring cold snaps can hurt new
seedings, too.
“Slope position, soil temperature, companion crop of oats, wind, all will influence
what has occurred in a particular field or part of a field,”î he said. “It is too early to
determine whether growing stem tips, crown and taproot damage has occurred.î”
If new seedings were permanently damaged, Barnhart said producers should
consider reseeding as soon as possible. He said to keep the good areas and drill
into thin or damaged areas, and that tillage may not be necessary.
“If you think that a cereal grain companion crop, still present, will be too
competitive or will impede the reseeding, then tillage may be required,Ӕ he said.
If regrowth shows frost or freeze damage, producers should wait a week to 10
days and dig some random plants, he said.
“Check whether remaining crown buds are still firm and intact. Split the taproots.
Healthy taproots are creamy-white in color, with a firm texture,î” he said “Freezeinjured taproots will begin to be ‘watery,’ tan/brown in color and beginning to
soften.Ӕ
If cold injury to established stands was light,î only affecting some of the early top
growth, determine if the growing point of the stems have been damaged,î
Barnhart said.
“If there was only leaf damage and the stem tip is recovering normally, follow
your normal harvest plans. If the stem tips are permanently damaged, you have
two choices: let the plant produce more branches and harvest a week or so later
than normal, now relative to the development of the new branches; or harvest the
damaged plants a few weeks early, with the understanding that it will be a
physiological stress to the plant,”î Barnhart said. “These, early cut plants, will
likely recover more slowly than normal and should be given an extra week or two
during one of the early summer regrowth cycles to recover their physiological
vigor.î”
If there has been widespread winter kill or severe injury, Barnhart said to
consider replanting a new alfalfa stand in an adjacent field.
Joel DeJong, Iowa State University Extension crops field specialist in
LeMars, said alfalfa crowns probably have had some damage, because the top
growth had already frozen off.
“It will be about a week before we can really assess how healthy those crowns
will be for regrowth,î” he said Monday. “Either way, we have slowed down the
advancement of the first crop of alfalfa. A few years ago, we had really cold
weather about this time in April, but I think the overnight temps were a little
colder. We lost a high percentage of alfalfa stands that year. Producers will need
to wait about a week, then dig crowns and look at the health of the crowns and
roots of alfalfa plants.î”